Selective catalyst reduction (SCR) devices are widely known and used, and are particularly used in combustion exhaust streams of diesel engines, as one example.
To meet Environmental Protection Agency (EPA) requirements, SCR devices are used to introduce a reductant which reduces NOx generated in engine exhaust systems. Along with SCR devices, sensory and feedback control capabilities have been employed to detect the presence of NOx and to introduce a reductant, based on the NOx detected by the control. The introduced reductant reacts with the NOx to facilitate its reduction. Feedback control modules can greatly improve NOx reduction or “deNOx” performance.
However, due to cross-sensitivity of sensors employed in known feedback control modules, NH3 (e.g. ammonia) that is present in the system also is read as NOx by the sensor (known as an “NH3 slip”). The NH3 slip is unwanted emissions caused by limited catalyst capability. When the control system generates a certain amount of NH3 slip, the NOx sensor reports the NH3 slip as NOx due to its dual sensitivity to NH3. As electrochemical sensors are typically used in such SCR devices in order to keep costs down, rather than optical sensors which are significantly more expensive (e.g. Fourier Transform Infrared Spectroscopy (FTIR) sensors), such cross-sensitivity of the sensor has been known to occur. The incorrect sensing information triggers false positive feedback, causing reductant introduction into the system at higher dosages, which can lead to an unstable system and reductant waste.
Thus, improvements can still be made to SCR devices, particularly those using a feedback control and improvements can still be made to limit overall reductant dosing.